As is known in the art, a power conversion circuit (or power converter) is an electrical or electro-mechanical device for converting electrical energy. One class of power converter is a dc-to-dc converter which converts a source of direct current (dc) from one voltage level to another utilizing inverting and rectifying devices.
As is also known, power conversion becomes more challenging for state-of-the-art low-voltage electronic circuits, including portable electronic devices, digital electronics, sensors and communication circuits among many items due to the desire of higher voltage conversion ratio and rapidly increasing power demand. The size and cost of the power conversion electronics (such as dc-dc converters) for these applications are also important, and can limit overall system design.
As is also known, a buck converter is a step-down dc-to-dc converter which utilizes a switched-mode power supply that uses multiple switches, an inductor and a capacitor. Prior art dc-dc converters implemented utilizing conventional buck converters have no transformation stage. Thus, inverting and rectifying devices in such converters are each exposed to both high current and high voltage signals. This makes such prior art approaches unattractive for high-conversion-ratio on-die dc-dc converters.
Referring now to FIG. 1, many conventional dc-dc converters 10 include a transformation stage 12 in addition to inverter 14 and rectification stages 16. DC-DC converter topologies having a transformation stage (e.g. coupled-inductor buck, flyback, etc.) primarily operate using only low-voltage rectifier devices. Furthermore, conventional converters having topologies which include a transformation stage requiring coupled magnetic circuit elements (e.g. transformers) which are difficult to manufacture on an integrated circuit die and which often have low efficiency. Moreover, inverter devices used in the aforementioned conventional dc-dc-converter circuits must be capable of fast operation while also being able to handle high voltage levels and are unavailable in circuits fabricated using conventional, commercially viable processing techniques.
The need for a step-down rectifier as introduced here is driven by the conversion ratio of conventional rectifiers for high-frequency rectification, which do not provide a desirable step down: from the ac voltage amplitude to the dc output. This places a greater burden on the transformation stage to achieve a large step-down, and the efficiency a matching network (or other transformer) is often inversely proportional to the voltage conversion ratio.
For example, and with reference now to FIG. 2, a traditional half-bridge rectifier 20 provides a π/2 voltage conversion ratio between an input fundamental ac peak voltage to the output dc voltage Vdc/Vac,pk (FIG. 2A). This step up in voltage in the rectification stage has the effect of decreasing the overall conversion ratio in a step down system. If the rectification stage could exhibit step-down characteristics, then the voltage transformation ratio in the transformation stage can be reduced while maintaining the same conversion ratio in the system as whole and hence improve the system performance. A full-bridge rectifier doubles the input voltage swing and gives a step-down voltage ratio of π/4. However, isolation is required for the full-bridge rectifier which increases the complexity of the system. As a result, a rectifier with higher step-down voltage conversion ratio is desired. It should be appreciated that while the half-bridge rectifier of FIG. 2 is here shown implemented with diodes, such rectifiers are more typically implemented with CMOS transistors acting as synchronous rectifiers.